Update back/forward navigation model to accurately follow HTML5 spec for state objects/hash changes
HTML5 specifies navigation and history traversal in terms of which document a navigation belongs to. This precise specification affects how hash changes and state object (https://bugs.webkit.org/show_bug.cgi?id=32052) traversals work, especially when scheduled during a page load (the page load shouldn't be cancelled if the scheduled navigation is to the same document)

Another thing covered by this bug is going back to a page that is in the page cache - activating page-cache items does not yet handle the popstate machinery, but having *all* back/forward history items track their document should help fix this.